The present invention relates to an electrophotographic photoconductor (also called "a photoconductor") and, more specifically, to a relationship between an electrophotographic characteristic and the radius of the, vacancy type defects which are present in the functional layer of a photoconductor, in which the radius is a value measured by a positron annihilation method.
Since the invention by Carlson disclosed in U.S. Pat. No. 2,297,691, numerous photoconductors have been developed including photoconductors using organic materials, such as phthalocyanines or azo compounds, as a charge generating material, as well as photoconductors using inorganic materials, such as amorphous silicon, a selenium-tellurium compound, or a selenium-arsenic compound.
In particular, a printer, a digital copier, a facsimile machine, or a digital image-processing complex machine capable of combined functions of these machines, occasionally uses a semiconductor laser emitting at a wavelength of 635 nm to 780 nm as an exposure light source for the photoconductor. For those apparatuses, photoconductors having sensitivity in that wavelength range have been developed. Phthalocyanines have been extensively studied as photoconductors for such a long wavelength light source because the phthalocyanines exhibit a large absorptivity in the wavelength range of a semiconductor laser as compared to other charge generating materials, and has excellent charge generating capability in that wavelength range. Photoconductors are known at present using phthalocyanines having a core metal of copper, aluminum, indium, vanadium or titanium, for example, as disclosed in Japanese Unexamined Patent Application Publication (KOKAI) Nos. S53-89433 and S57-148745, and U.S. Pat. Nos. 3,816,118 and 3,825,422.
Further, phthalocyanine oligomers are known, which show an excellent photoconductive characteristic in that wavelength range, such as .mu.-oxo-Ga(III) phthalocyanine dimer and .mu.-oxo-Al(III) phthalocyanine dimer. These have been studied for photoconductor application as disclosed by Yamazaki et al. in Nihon-Kagakukai-shi (Journal of The Japanese Chemical Society) 1997, No. 12, p. 887.
In the apparatuses equipped with a white light source such as an analogue copier, a photoconductor in the mainstream uses the charge generating material of a bisazo compound, which has sensitivity in the light wavelength range between 400 nm and 650 nm.
A photoconductor of a so-called function-separated type comprises a charge generation layer and a charge transport layer formed on an electrically conductive substrate optionally through an undercoat layer that is formed if required. The charge generation layer comprises a resin binder and particles of above-described pigments dispersed in the resin binder. The charge generation layer has a thickness of about 1 .mu.m or less. The charge transport layer is formed by dissolving a charge transport material of relatively low molecular weight in a resin binder including polycarbonate resin for example, to become a so-called molecular dispersion state. The film thickness of the charge transport layer is generally in the range between 10 .mu.m and 30 .mu.m. The charge transport material has a partial structure for serving charge transport function, such as butadiene-, triphenylamine- or hydrazone-structure. In addition, an alkyl group or a polar group like a halogen atom is often introduced into the charge transport material. The alkyl group provides compatibility with the resin binder and a solvent. The polar group serves to adjust electronic properties, such as ionization potential, that relate to charge transport ability of the material. In recent years, a resin binder has been proposed, in which a molecular structure having charge transport function is introduced into a side chain or a portion of a principal chain of the resin binder so that the binder itself serves a charge transport function while maintaining its mechanical characteristic of a binder.
The charge transport layer of conventional photoconductors suffers from deterioration of electrical characteristics, for example increase of residual potential, due to charge trapping at the trapping sites existing in, for example, the intermediate product included as impurities of synthesis process of the charge transport material, the residue of the catalyst used in synthesis process of the charge transport material or the binder resin, or the photocomposition product produced by light exposure of the photoconductor in use or of the coating-liquid in the manufacturing process. The charge trapping could also be caused by inhomogenuity in the molecular structure of the binder resin, namely, inhomogenuity of the folding structure of the principal chain, or variation of the end groups of the resin. Minute voids in the resin could also be sites of electrical trapping.
A photoconductor commonly requires good charging capability, small attenuation in the dark, and a small residual potential. Further, these properties should not change in repeated use. However, when the combination of the resin and the charge transport material is so improper that a large number of above-described defects are generated, such electrical characteristics cannot reach the required level. While a photoconductor using a phthalocyanine compound as a charge generating material, in particular, exhibits high sensitivity in the high wavelength range as described earlier, such a photoconductor has a disadvantage in that the charged voltage is low in the first turn and stabilizes on the second and later turns. When such a photoconductor is used in a process that utilizes the first turn for image formation, non-uniformity in the printed image occurs due to low charged potential in the first turn.
With the increase in processing speed of CPU and data transfer rates in recent years, higher printing speed is needed. Therefore, utilization of the first turn of the photoconductor for image formation has become an important subject for high-speed start-up of a printer and a digital copier. The above-described instability of charged potential may be attributed to the following mechanism. The charges generated in the dark in the charge generation layer are trapped at the trapping sites as mentioned earlier in the interface between the charge generation layer and the charge transport layer, and in each of the layers in the photoconductor. The trapped charges are released in the first turn of the charging period, resulting in excessive cancellation of the surface charges, which leads to a lower charged potential. Therefore, reducing the density of the trapping sites is an important technical target for preventing the unsteady charging characteristic.